To draw the correct Lewis structure for carbonic acid (H₂CO₃), we start by determining the total number of valence electrons available. Carbon (C) contributes 4, oxygen (O) contributes 6 each (there are 3 O atoms), and hydrogen (H) contributes 1 each (there are 2 H atoms). This gives us:
- Valence electrons from C: 4
- Valence electrons from O: 6 x 3 = 18
- Valence electrons from H: 1 x 2 = 2
Adding these together, we have a total of 4 + 18 + 2 = 24 valence electrons.
Now, we can start sketching the structure:
- Place the carbon atom in the center since it is the least electronegative.
- Attach the three oxygen atoms to the carbon. Two of these will be bonded to the carbon with single bonds, and one will be bonded with a double bond to satisfy the valency of carbon and to achieve a stable structure (the double bond will be with one of the oxygens).
- Next, we add the two hydrogen atoms. Each hydrogen will bond to one of the singly-bonded oxygen atoms.
The resulting structure looks like this:
In this structure, the carbon atom has 4 bonds (two single bonds to the oxygen atoms each connected to a hydrogen, and one double bond to the other oxygen), fulfilling its tetravalency. Each oxygen atom also fulfills its valency by having either two or four bonds altogether.
This Lewis structure helps to visualize how electrons are shared in carbonic acid, showing us the stability and arrangement of its atoms.